2, Vanderbilt University, Nashville, Tennessee, United States
The ability to controllably position single atoms inside a material is a prerequisite for the creation of next generation atomic-scale devices. Recently, there has been interest in using scanning transmission electron microscopes (STEMs) as tools for atomic-scale fabrication. Advances in aberration correction allow for precise control over the size and position of the electron probe. Recent work has demonstrated the ability to control “beam damage” to modify materials in a way that may prove beneficial to the overall material properties. This ability to directly manipulate materials at the atomic scale offers new opportunities for quantum engineering, such as qubits based on individual donors in a semiconductor. The current state of the art in manipulation on the atomic scale includes the positioning and imaging of single atoms using scanning tunneling microscopy (STM) or atomic force microscopy (AFM). However, the movement of matter by these techniques is limited to two-dimensional surfaces. In order to achieve quantum device fabrication, it is necessary to be able to controllably position individual atoms within a three-dimensional crystal, but previously there existed no method for targeted manipulation of a specific atom within a bulk structure.
Here we demonstrate for the first time the ability to controllably move and place subsurface bismuth dopants in a silicon crystal at room temperature using STEM. Bismuth has advantages over phosphorous for donor-based quantum computing, and its large atomic number relative to Si makes this system an ideal test case for single-atom manipulation in STEM. The ability to controllably move Bi dopants is indicated by two findings from our density functional theory calculations: (1) the strain induced by a substitutional Bi dopant allows for Si vacancies to be preferentially created adjacent to the Bi atoms, and (2) there is no significant energy barrier for the Bi atom to hop into these vacancies once they have formed. Using the electron beam, we exploit this vacancy-mediated motion to direct and place Bi dopants below the surface in specific columns within an oriented Si crystal, an important step towards creating a functional quantum device. This result represents a promising advance in atom-by-atom fabrication, enabling the tuning of material properties through controlled dopant positioning.
This work was sponsored in part by US Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division (ARL) and by the Laboratory Directed Research and Development Program of Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the U. S. Department of Energy (BMH, JS, PCS). DFT calculations (HS, STP) were supported by Department of Energy grant DE-FG02-09ER46554 and were performed at the ERDC DSRC with support from subproject AFSNW32473012.